For providing electric energy to an utility grid an electrical power production plant may comprise one or more converter modules (also called inverter modules) for converting a variable frequency power signal (or voltage or current) to a fixed frequency (e.g. 50 Hz or 60 Hz) AC signal (or voltage or current). In a typical converter module the variable frequency AC power signal may first be converted to a DC signal (or voltage or current) using a number of power transistors. Further, the DC signal (or voltage or current) may subsequently be converted to a fixed frequency AC signal (or voltage or current) using one or more (in particular six for a two level three phase inverter) power transistors—see what FIG. 6 for a basic example.
The power transistors connected to the DC signal (or voltage or current) (also called DC-link) may be controlled regarding their conductivity by corresponding control circuits in order to deliver a three phase AC signal (or voltage or current) at three output terminals of the converter module. In particular, the converter module may be implemented as an electrical semiconductor power conversion unit.
It may be known that power semiconductors in power conversion units are exposed to induced over-voltages during turn off transitions. Notice that this phenomenon may not be limited to the inverter topology shown in FIG. 6, but it may be a generally recognized problem in the field of switching converters, due to unavoidable impedance in the switching power circuit.
The essence of the problem may be that to switch-off a power semi-conductor device may generate through the leakage inductance of the inverter power circuit a transient overvoltage in the micro-second to sub-microsecond time domain. This overvoltage, if not controlled, may lead to breakdown of the power semi-conductor device which can then lead to catastrophic failure of the power semi-conductor device. Techniques to control this power semi-conductor switch-off related over-voltage include the use of Zener or transil type diodes connected between (say) the collector and gate of (for example) an IGBT device (as an example of a power semi-conductor device).
Then looking to the application conditions of the power semiconductor device in the inverter module (or particularly the active rectifier mode of operation of the inverter module), if the power flow and associated current from inductively fed ac terminals is terminated, then to bring the current to zero in the feeding inductors connected to the ac terminals of the inverter (operating in active rectifier mode) (for example the leakage inductance of a permanent magnet generator in the example of a direct drive wind turbine) then sufficient volt seconds have to be applied to the leakage inductance (reset voltseconds as per text book descriptions). All the time that the reset is taking place, energy is being transferred from the leakage inductance to the DC link circuit of the inverter operating in active rectification mode. Further energy is also received from the shaft of the generator (and before that the blades of the turbine and the wind) which is a function of the back-emf (electromagnetic force) of the generator during the reset period and the generator current waveform during the reset period. In a typical scheme, the time to reset the current in the leakage inductance of the generator to zero will be in the order of 10 ms-20 ms, and a significant amount of energy is transferred from the generator to the DC link during this event.
If, in a typical wind turbine power converter, the DC link is connected to a network inverter to further process the power received from the permanent magnet generator and transfer this to the power network or power grid, then the DC link voltage does not rise significantly, except for the initial say 1 μs voltage spike associated with the turn-off of the power semi-conductor device.
However, if the reason for turning off the power semiconductor devices of the generator bridge are due to the non-availability of the network as a receiver of this energy, then the energy received from the generator during the turn-off event (resetting in the generator leakage inductance current to zero) may charge up the DC link capacitance. Depending on the dimensioning of the DC link capacitance, the ultimate DC link voltage may significantly exceed the operational threshold of the Zener or transil clamp network necessary to control the overvoltage during the turn-off of the power semi-conductor device. If a countermeasure was not included, then the Zener or transil clamp network would result in the catastrophic damage being caused to the entire inverter system as the Zener or transil clamp network would force the power semi-conductors into a conductive state and expose the power semi-conductor devices to energy far in excess of their dissipation capability.
In particular, direct drive permanent magnet generators may generally feature high leakage inductance compared to equally rated induction machines. During certain fault scenarios the amount of energy stored in that leakage inductance may become very high due to very high currents. When this energy is subsequently transferred to the DC link capacitors (which may represent energy storage elements), the DC link voltage may rise to a high level. In certain worst case scenarios the DC link voltage may rise significantly above the clamping level and finally the clamping scheme becomes a threat rather than a protection mechanism, because the semiconductors will then dissipate energy in excess of their capability and may be damaged.
Thus, over-voltages occurring in particular at a DC link of a converter module may be harmful for components, in particular power transistors, comprised in the converter module. Thereby, the power transistors, or generally controllable switches, may be destroyed due to the over voltage.
To manage this problem, the conditional active clamp circuit of this invention is described.
There may be a need for a circuit and for a method for protecting a controllable power switch, in particular a power transistor, which is connected between terminals to which a voltage is applicable. Further, there may be a need for a circuit and a method for protecting a controllable power switch which enables protection of the controllable power switch and clamping of the voltage between terminals of the power switch. Further, there may be a need for a circuit and a method for protecting a controllable power switch, wherein voltage clamping may be performed more effectively.